Layer Ingredients: Lightly grease the slow cooker. Lay the chicken breasts flat on the bottom. Top evenly with sliced mushrooms and minced garlic.
Add Liquids: Pour the Marsala wine and chicken broth over the top. Season with salt and pepper.
Slow Cook: Cover and cook on low for 5 to 6 hours (or high for 3 to 4 hours) until the chicken is cooked through and tender.
Thicken Sauce: 30 minutes before serving, whisk the cornstarch and cold water together until smooth. Stir the slurry and the heavy cream into the slow cooker.
Finish: Turn the cooker to high and leave uncovered for 20 minutes until the sauce thickens. Garnish with parsley and serve over noodles.